1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus, a scanning lens, and a molding method of the scanning lens, and more specifically, to an optical scanning device for scanning a surface to be scanned using a light beam, an image forming apparatus equipped with the optical scanning device, a scanning lens molded by resin injection, and a molding method of the scanning lens by injection molding.
2. Description of the Related Art
In recent years, advancement has been made in a high speed color printing method of an electrophotographic image forming apparatus used in a laser printer, a laser plotter, a digital copying machine, a laser facsimile, or a multifunction peripheral including the same, and a tandem-type image forming apparatus has prevailed by including a plurality of (normally four) photosensitive elements serving as image carriers.
In the tandem-type image forming apparatus, the photosensitive elements are disposed along a conveying direction of a recording sheet. A plurality of light beams emitted from a plurality of light sources corresponding to the respective photosensitive elements is deflected by a light deflector to simultaneously expose the surfaces of the respective photosensitive elements through a corresponding scanning optical system (scanning image-forming optical system), thereby to form respective latent images thereon. These latent images are made visible with developing units using developers of different colors such as yellow, magenta, cyan, and black, sequentially superimposed and transferred onto the same recording sheet, and fixed, so that a color image is formed thereon.
However, the tandem-type image forming apparatus requires a plurality of light sources with the number thereof corresponding to that of the photosensitive elements, and accordingly, there has been an inconvenience in that an increase in the number of components raises a manufacturing cost and enlarges the device.
A proposal has been made for a common use of the scanning lens by a plurality of light beams directed toward the photosensitive elements that are different from each other (see, for example, Japanese Patent Application Laid-open No. 2002-160268).
A proposal for reducing the cost has also been made for the light deflector, on a deflecting reflective surface on which light beam is obliquely incident with respect to a sub-scanning direction (see, for example, Japanese Patent Application Laid-open No. 2003-5114).
However, in the obliquely incident optical system, there has been an inconvenience in that the light beam directed to a peripheral image height in the vicinity of an edge of an image forming region on the photosensitive element, in particular, is incident on a scanning lens in a twisted state, which may increase a wave aberration. In this case, the optical performance at the surface of the photosensitive element significantly degrades, the diameter of the beam spot becomes large, and higher image quality is inhibited. Furthermore, a color shift may occur when the toner images of the respective colors are superimposed because the magnitude in the bending of a scanning line differs from one light beam to another depending on an angle of oblique incidence of the light beam.
A method for correcting the degradation of the optical properties specific to the obliquely incident system on the surface of the photosensitive element has been proposed (see, for example, Japanese Patent Application Laid-open No. 2006-72288). In the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2006-72288, an optical surface that does not have a curvature in the sub-scanning direction but has an eccentric tilting in the sub-scanning direction changed toward a main scanning direction is disposed between another optical surface that is most refractive in the sub-scanning direction and the light deflector, thereby enabling to correct the wave aberration. Furthermore, the bending of the scanning line can also be corrected by providing a scanning lens with a surface which is similar to the optical surface described above on a scanned side thereof. As a result, the degradation in the optical properties specific to the obliquely incident optical system can be corrected, and a cost reduction and miniaturization are achieved compared to the prior art.
Japanese Patent Application Laid-open No. 2008-15139 discloses an optical scanning device configured by an image-forming optical element that includes a plurality of optical effective portions, and one or more optical non-effective portions that does not allow the light beam to pass therethrough and that are provided between the plurality of optical effective portions such that a distance in an optical axis direction from a line segment connecting each apex of the plurality of optical effective portions to a apex of the optical non-effective portion in a sub-scanning cross-section is smaller than or equal to 1.0 mm.
Generally, a scanning lens molded by resin injection to a mold having a cavity of a shape similar to the scanning lens is used from the viewpoint of the manufacturing cost and the precision control in forming a shape of an optical surface.
In recent years, there is a need for further reducing a price of an image forming apparatus and achieving higher image quality on an output image thereof, and accordingly, there is also a need for further reducing a price and improving scanning accuracy of an optical scanning device.
An attempt has been made to use a scanning lens having a shape in which a plurality of optical surfaces is disposed along the sub-scanning direction was used with the aim of reducing the number of components; however, with the image forming optical element disclosed in Japanese Patent Application Laid-open No. 2008-15139, the accuracy in the shape of an optical surface has been found insufficient and it has been found difficult to improve the scanning precision.